Ionic homeostasis and volume regulation in fish cells

All types of cells respond to changes in environmental osmolarity with alterations in cell volume and try to restore cell volume by activation of various transport mechanisms. This project tried to elucidate three fundamental aspects of this phenomenon in cells from freshwater fish. First, this study addressed the question if cell volume changes interact with metabolism and the physiological meaning of these interactions. Second, we wanted to elucidate the mechanisms involved in the regulation of cell volume and the signalling pathways engaged. And third, we studied the activation and expression patterns of the signalling pathways playing a role in volume regulation. The experimental models applied for these studies were freshly isolated liver cells from two fish species of different life style and ecology, cultures of these liver cells, a trout hepatoma cell line, and a trout gill cell line. Using cells from different species and comparing their responses to osmotic perturbations we wanted to elucidate phenomena specific to cells with different capabilities to cope with environmental stress. Our results showed that volume changes may in fact interact with metabolism and provided a detailed analysis of the transport mechanisms involved in volume adjustments of fish cells. Although these responses were in part similar to responses of cells from mammals, they also showed some interesting differences, suggesting that cells from fish have adopted specific mechanisms to cope with their aqueous environment. Furthermore, we elucidated some of the signalling pathways controlling specific transport mechanisms, the role of the cytoskeleton, and some of the signalling cascades linking changes in cell volume and regulatory responses. Signalling pathways responded in a stimulus-specific and cell type specific manner and this, again, partly deviated from response patterns observed in mammals. Overall, our project provided new insight into cellular events triggered upon changes of environmental conditions and showed that cells from lower vertebrates, such as fish, are a useful model for the investigation of metabolic and ionic adjustments and signalling pathways underlying these responses.